Use of mathematical models requires the estimation of model parameters, which is usually known as the calibration of the model. In general, parameter optimization is preferred in model calibration to the trial-and-error visual comparison of observed and modelled output responses, due to subjectivity and the time-consuming nature of the latter approach. An optimization procedure, called two-stage inner/outer optimization, is described in this paper, which can be used to estimate the model parameters of any urban stormwater drainage catchment modelled with any urban drainage computer modelling software. However, the ILSAX computer software was used in this study. The method is designed to provide the 'best' set of model parameters that consider several storm events simultaneously. Impervious area parameters are obtained from frequent 'small' storm events, while the pervious area parameters are obtained from less-frequent 'large' events. The Giralang catchment in Canberra (Australia) was used to demonstrate the method. Several 'small' and 'large' storm events of the catchment were considered in parameter optimization. Few other storm events, which were not used in model calibration, were used to validate the model parameters obtained from calibration. Results from both calibration and validation showed that the 'best' set of model parameters obtained for the catchment was able to produce hydrographs similar to the observed hydrographs. Pervious and impervious area parameters obtained from calibration agreed well with the information gathered from other sources such as aerial photographs and published literature.

Stormwater drainage models are increasingly being used in design and analysis of urban drainage systems. If these models are to be used for ungauged drainage systems for which no storm and corresponding flow data are available, then the model parameters have to be estimated through regional equations. These regional equations define the model parameters via measurable catchment properties. In this study, regional equations of two impervious area parameters, namely directly connected impervious area percentage (DCIA) and its depression storage (DS i ), were developed for use in urban drainage models, using 15 gauged urban catchments in Melbourne Metropolitan area (Australia). The 'small' storm events of these gauged catchments were first used to calibrate the impervious area parameters, since the 'small' storm events produce runoff only from the impervious areas. These calibrated impervious area model parameters were then analysed with several measurable catchment properties to derive the regional equations. The results of the study revealed that DCIA was depended only on household density of the catchment, while DS i was not correlated with any of the catchment properties analysed.